In the field of wireless communication, communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system, such as a Wireless Local Area Network (WLAN), or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone, between a terminal and an Access Point/Access Node (AP/AN), and/or between a terminal and a server via an access network and possibly one or more core networks, comprised within the communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the access network, such as a Radio Access Network (RAN), with another entity, such as another terminal or a server.
The communications network covers a geographical area which is divided into geographical subareas, such as coverage areas, cells or clusters. In a cellular communications network each cell area is served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used to denote the transmission path from the base station to the mobile station. The expression Uplink (UL) is used to denote the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic.
Dual Connectivity (DC) is a feature that has been standardized in 3GPP for LTE. In Dual connectivity, a user equipment (UE) can connect to two evolved Node Bs (eNBs) concurrently. One of the two eNBs, called a Master eNB (MeNB), is the signaling anchor point and the other one of the two eNBs, called a Secondary eNB (SeNB), is used to further increase user throughput. In DC, the frequency used by the MeNB and the SeNB, respectively, is assumed to be different. This is schematically illustrated in FIG. 1, wherein the UE is connected to the Master eNB on a first carrier carrier1 and to the Secondary eNB on a second carrier carrier2.
Further, in DC, there are three types of radio bearers; a Master Cell Group (MCG) bearer, a Secondary Cell Group (SCG) bearer and a Split bearer. The MCG bearer is a bearer served by the MeNB, the SCG bearer is a bearer served by the SeNB, and the Split bearer is a bearer served by both the MeNB and the SeNB. FIG. 2 schematically illustrates a combined protocol architecture according to the prior art, wherein the radio bearers between the PDCP, the RLC and the MAC layers to the left in the figure (dotted lines) are served by the MeNB, the radio bearers between the PDCP, the RLC and the MAC layers to the right in the figure (dashed lines) are served by the SeNB and the radio bearers between the PDCP, the RLC and the MAC layers in the middle are served by both the MeNB and the SeNB.
Further, in DC, when the MeNB needs to be changed to another eNB, whether it is the SeNB or another eNB, the signaling procedure, i.e. the steps 301-316, illustrated in FIG. 3 has to be followed. That is, the Serving SeNB (S-SeNB) needs to be released first, cf. step 303, then the UE attaches to the Target eNB (T-eNB), cf. steps 305-306, and thereafter the new SeNB, cf. steps 311-312, can be added.
The 5th generation mobile networks or 5th generation wireless systems (5G) is the next major phase of mobile telecommunications standards beyond the current 4G/IMT-Advanced standards.
The Next Generation Mobile Networks Alliance defines several requirements for the 5G communications networks. For example, should data rates of several tens of megabits per second be supported for tens of thousands of users. One (1) gigabit per second should be offered simultaneously to tens of workers on the same office floor. Several hundreds of thousands of simultaneous connections should be supported for massive sensor deployments. The spectral efficiency should be significantly enhanced as compared to 4G. The coverage should be improved, and the signalling efficiency should be enhanced. Further, should the latency be reduced significantly as compared to LTE. Furthermore, Critical Machine Type Communication (C-MTC) is also an important use cases in 5G, which has the ultra low latency requirement, and the latency requirement is usually tightly connected to very ultra-reliability requirements.
NeXt generation (NX) is envisioned to be a non-backward compatible system in 5G. In NX, the frequency range targeted includes 1 GHz up to 100 GHz. Especially due to the addition of high frequency spectrum to the wireless operation, high gain beamforming is a “must” to compensate the negative effects due to unfavorable radio propagation properties. One issue with high gain beamforming is that the serving beam is optimal only for a small area, and expectedly a small fraction of time. When the UE moves, the serving beam can deteriorate very fast which makes the mobility in NX a challenge. In order to solve this issue, a cluster concept is proposed for NX, where multiple access nodes can form a cluster to serve UE together. When the UE moves, if the beam from one access node deteriorates, other beams from other nodes can ensure that the UE can still be served and receive good performance. FIG. 4 schematically illustrates a cluster in an NX communications network. Especially, FIG. 4 schematically illustrates a UE-specific serving cluster 400, wherein a group of ANs that are located in the vicinity of a UE 402 are ready to serve the UE 402. In FIG. 4, the exemplifying group of ANs comprises a Principal Serving Access Node (P-SAN) 404 and two Assisting Serving Access Nodes (A-SANs) 406a, 406b. The P-SAN 404 is responsible for maintaining the connection between the severing cluster 400 and the UE 402, and it is intended to be the main serving AN for a given UE 402. Data blocks associated with the UE 402 are communicated directly through the P-SAN 404 most of the times. To control the fast beam switch, a cluster head 410 is needed for the coordination in the cluster 400. When the UE 402 moves, the standby AN 408a, 408b may also be added to the cluster 400. The standby AN 408a, 408b may be added as P-SAN or A-SAN.
However, it is a problem with current procedures for switching master node for the UE, such as the procedure according to FIG. 3 described above, that they are so time consuming whereby requirements on low latency cannot be fulfilled. Further, the signaling required for accomplishing the switching results in high usage of radio resources which may cause interference and deteriorate the performance in the communications network.